GRATICULES

125

practical experience during the War has frequently led to complicated patterns being successively simplified.

With regard to the lino thickness of graticules, a number of interesting little points require to bo borne in mind and balanced, and in considering these it is convenient to remember that a lino 1/1000" wide, viewed with the unaided eye at normal, i.e., 10" distance, subtends an angle of about 20 seconds. Opaque linos when viewed against a white ground appear rather thinner than they actually are, as the white light encroaches on the two edges of tho line, by irradiation of the eye. When tho opaque lino is too narrow, the lino appears faint and indistinct and begins to bo uncomfortable. With tho unaided eye a lino 1/500" or 1/20 mm. is

irradiation. It is rarely necessary or advisable to go below -001" in width, tho more so as they have the advantage that tho object viewed can bo seen through thorn, and centred to tho middle of the transparent line with very great accuracy. This feature is also useful because lines of varying thickness .can bo used on tho same graticule disc for differentiation purposes, without sacrificing accuracy of measurements. Whoro squared lines or repeating patterns, or markings in the nature of massed patches, are required, transparent linos should always bo given preference over opaque ones, unless tho latter arc indispensable for reasons necessitating maximum contrast. Tho depth of tint of transparent lines should bo appropriate to the magnification under which they arc to bo viewed, as it is a somewhat remark-

Fid. 3.—Bluck oiMiquo. Lino thickness -001". Magnification 85.

sufficiently narrow for comfort. For eyopioco graticules the magni filiation of tho oyo Ions has to bo taken into account, and the boat rule for comfort is not to have linos narrower than needful. With eyepieces magnifying x 5 to x 8 opaque linos 1/2000" or 1/80 mm. will bo found sufficiently fine. In micrometer scales divided into tenths or twentieths of a millimotro the width of the linos may reasonably bo reduced to 1/5000" or 1/200 mm., and in scales of this description, or scales divided oven more finely for microscope stage micrometers of tho ordinary description, opaque lines are the most suitable. The illustrations, Figs. 1 to 4, aro from photomicrographs of Rlioinborg Filmloss Graticules taken by Mr. J. E. Barnard, F.Inst.P.

Transparent lines seen against a white ground always appear markedly narrower than opaque ones of similar width, duo partly to the greater encroachment on their edges by

Fio. 4.—Bluck opacnio. Lino, thickness -001,". Magnification OfiO.

able fact that it differs with the magnification. Tho same grey line at -002" width may look almoHt black to tho unaided eye; aeon with on ocular x 5 it appears grey, with an ocular x 10 a lighter grey, on the microscope stage-magnified 100 times it appears a very light groy indeed.

Differentiation for the sake of ctoai'noss and •faciliby in reading or counting is a factor which should receive consideration. For instance, in a graticule for counting purposes, a field consisting of alternate transparent grey and colourless square patches in a chessboard pattern,—the so-called Chessboard Micrometers —are more comfortable and easy to work with than graticules simply divided into squared lines. Linos of different thickness on tho same graticule are another form of differentiation. Oases ocour, however, where it is still more convenient to differentiate by lines of different colour on tho same graticule. Such graticules